432 THE PRODUCTION OF SULPHUR DIOXIDE

dilute acid containing sufficient organic matter to decompose
the acid on subsequent heating. The SO2 thus obtained is
purified and reoxidised to attain sulphuric acid.

H. PREPARATION OF SULPHUR DIOXIDE IN THE

PURE OR LIQUID STATE*

Formerly pure sulphur dioxide, free from nitrogen and
excess of oxygen, was required only in very few cases for
industrial purposes. The methods employed for preparing it
were various, one of the commonest being the action of con-
centrated sulphuric acid upon copper at a higher temperature.
This is, of course, only applicable where there is a sale for the
cupric sulphate formed, and is, moreover, hardly workable on a
large scale. Cheaper and easier is the process of heating strong
sulphuric acid with charcoal, when a mixture of SOS with COS
(and CO) is obtained :

2H2S04 + C« 2HaO + 2SOS+ COr

The CO and CO2 are harmless in many applications of S()s.

Sulphur dioxide, quite free from other gases, is made by
heating concentrated sulphuric acid with sulphur ;

this can be done by running a slow stream of sulphuric acid on
sulphur, heated to about 400° in an iron retort It should,
however, be noticed that melted sulphur acts very strongly
upon cast-iron ; hence the operation should be performed in a
cast-iron pan widening out at the top so that a lining of acid-
resisting bricks can be put in It Concentrated sulphuric
is boiled with sulphur, which floats on the top and m by
the brick lining from coming into immediate with the
iron, whilst the lower part of the pan is fully to the
heating action of the fire
Cast-iron pans are, however, used at the
without such linings, but in this case they are 3 in.
in thickness, and require rather frequent renewal,
Sulphur dioxide was made by the Compagnie
des proo§d&> Raoul Pictet (Ger. P. 22365), and was In
1 A special treatise on the preparation, and cif
pure sulphur dioxide is FlBssiges Schwtfsldioxydi by A.
1:900),